Microelectromechanical systems (MEMS) have been shown to be useful for a variety of consumer, industrial and military applications. Most MEMS devices are fabricated on semiconductor substrates (e.g., silicon, Gallium Arsenide, Silicon-On-Insulator, etc.) using standard Integrated Circuit (IC) processes in combination with specialized micromachining processes. Collectively these manufacturing technologies are frequently called microfabrication processes.
Conventional MEMS processes which are performed on silicon or other semiconductor substrates can lower the cost of products, but not to the extent required for many consumer or industrial applications. Typically, MEMS devices are batch fabricated, either as discrete components or directly on or within integrated circuits (“IC”) as a part of a merged MEMS-IC process. Although both approaches can potentially lower cost somewhat, the reduction is not sufficient for many applications. Even if a sufficiently low cost process for the fabrication a MEMS device can be achieved, the manufacturing of MEMS devices can incur significant additional costs associated with packaging and integration, resulting in an expensive overall system or product cost. The fabrication of MEMS devices directly on or within integrated circuits (“integrated MEMS”) requires expensive process development as well as some compromises in device performance. Both approaches are also limited by total processing area, and the resultant gains from these approaches are modest at best. Consequently, one of the key limitations of MEMS technology has been the cost of manufacturing MEMS devices and systems using semiconductor substrates and microfabrication process technologies. Another limitation relates to the high cost associated with packaging these devices and systems. Yet another limitation relates to the cost and difficulty of realizing systems wherein MEMS and microelectronics are combined into modules or integrated together to form functional systems. Therefore, there is a tremendous need for more functional and cost-effective fabrication, packaging and integration techniques for implementation of MEMS-based RF devices.
Recently there has been a large interest in making MEMS Radio Frequency (RF) devices and systems for a variety of high volume communication applications. Although MEMS-based RF components and systems have been demonstrated, all have been realized on traditional semiconductor materials, primarily silicon wafers. While this approach works for the demonstration of a device, it has several severe disadvantages for the performance and potential commercialization of RF and microwave devices. First, the dielectric losses of the silicon substrate are very high at frequencies above 1 GHz. Second, the cost of silicon substrates and processes used to fabricate MEMS RF devices on these substrates are too high compared to existing technologies. Third, the packaging costs of silicon and other semiconductor material based MEMS devices are very high, particularly for devices that must operate at high frequencies and under extreme environmental conditions.
While the losses of the silicon substrate can be reduced appreciably by selectively removing the silicon from under the active devices and the associated signal paths using an isotropic etchant, such as Xenon Diflouride (XeF2), this is an expensive process and one that is not readily compatible with the fabrication of active MEMS devices. Consequently, the resultant manufacturing yield will be low and the cost will increase appreciably. Other semiconductor substrates having lower dielectric losses can be employed for the fabrication of MEMS devices, such as Gallium Arsenide (GaAs), resulting in high performance devices. However, the cost of these materials and the costs to fabricate devices on these materials are typically two orders of magnitude higher than even silicon wafers and processes. Consequently, the resultant device or system cost will be far too high for many consumer or industrial applications. Furthermore, any semiconductor based MEMS device will require a separate packaging technology that will need to be specifically developed to meet the demanding requirements of a commercial product. Packaging techniques that can meet the required specifications and simultaneously provide a sufficiently low cost have not been readily available in the past.
Nevertheless, there is enormous opportunity for MEMS technology in the application of RF and microwave devices and systems. If the cost and performance goals can be met, the potential market sizes for these devices will be enormous. However, in order to exploit this opportunity, there is a need for a new low-cost material that has low dielectric losses at high frequencies and onto which MEMS devices can be successfully fabricated with high yield. Furthermore, there is a need for the capability to suitably and inexpensively integrate these MEMS devices with other MEMS device and components to form functional systems. There is also a need to suitably and inexpensively package MEMS devices and systems. It is useful to elaborate in some detail about specific RF and microwave MEMS devices and systems that can greatly benefit from fabrication, packaging and integration that can be performed on a low-cost low dielectric loss material.
MEMS Radio Frequency (RF) switches have been shown to have very low dielectric losses at very high frequencies, if fabricated on specialized substrate materials such as GaAs. Compared to traditional active microwave switches (based on active components such as transistors or diodes), the quality factors (where the quality factor is given by 1/Ron Coff, where Ron is the resistance of the switch in the ON-state and Coff is the OFF-state capacitance of the switch) of these MEMS RF switches are very high. Therefore, MEMS RF switch components have the potential for use in many types of applications. However, these devices have been limited due to the extremely high cost of fabricating and packaging the MEMS switches on Gallium-Arsenide substrates.
An important application of MEMS switches is as phase-shifters in phased-array antennas. Traditionally, phase-shifters for phased-array antennas have been implemented using active electronic components (e.g., transistors, diodes, etc.) made from exotic and expensive materials such as Gallium-Arsenide (GaAs). Typically, even at high volume production, GaAs-based active phase-shifters can cost more than 100 times to fabricate and are more than twice as lossy as MEMS phase-shifters. As a result, phase-shifters are the main cost driver in phased-array antennas. If active electronic GaAs phase-shifters are employed, about 45% of the overall cost of a receiver antenna system can be attributed to the phase-shifters alone. This is attributed to the high fabrication cost for such devices and the additional cost associated with the amplification and thermal management required by their high losses at their operational frequencies. Consequently, due to the high cost, active electronic GaAs-based phase-shifters have been limited to use in military array antennas.
For similar reasons, a variety of other MEMS RF components and systems, including: electronically-tunable variable capacitors, closed-loop controlled variable capacitors, tunable inductors, tunable LC filters, tunable LC networks, reconfigurable RF antennas, phased array antennas, as well as combinations of the above MEMS devices and systems, would greatly benefit from the ability to be fabricated on a low-cost, low dielectric loss material. Furthermore, these devices and systems would also benefit from low-cost and high performance approach for packaging and integration.
Recent developments in Low Temperature Co-Fired Ceramic (LTCC) processing, combined with the recent availability of new high-quality LTCC substrate materials having low dielectric losses at high frequencies, have made it possible to fabricate, integrate and package RF MEMS devices and systems with high performance and at low costs.
Direct fabrication of MEMS RF components onto LTCC substrates is key to reducing the cost of these systems, while simultaneously achieving high functional performance. The use of LTCC substrates and processing techniques to integrate and/or package MEMS RF devices also provides many advantages. The cost to attempt to integrate different types of MEMS RF devices together or with microelectronics is enormous. This is because the processing steps used to fabricated the merged devices or systems greatly influence material properties and resultant device performance. Using LTCC as a substrate material greatly simplifies and lowers the cost of integration.
With respect to packaging, it is frequently the case that the cost of packaging MEMS devices is more than the cost of the MEMS device itself. This is because the package must be specifically designed and manufactured for each individual MEMS device type. Furthermore, the package must protect the MEMS device from the environment, but simultaneously allow the MEMS device to interact with the environment. The use of LTCC as a substrate material that can provide electrical connections through layers, as well as methods to make MEMS directly on the LTCC material, and methods to affixed semiconductor substrates with high quality electrical connections to the activated components, while also providing suitable packaging protection, and at low cost, is a significant improvement in MEMS technology.
The present invention, which uses MEMS RF devices fabricated on a LTCC substrate, allows the cost and difficulty of realizing a system to be dramatically reduced so that MEMS RF devices can be more broadly used for consumer and industrial applications. The present invention, which uses an LTCC substrate with electrical connections across or through multiple layers of the LTCC substrate, and when bonded or affixed to MEMS on LTCC substrates or other substrates such as semiconductor ICs, enables the device or system to be integrated and packaged with a significant reduction in cost so that products based on MEMS technology can be used more broadly for consumer and industrial applications.
The present invention, which uses MEMS RF switches fabricated on an LTCC substrate, allows the cost of MEMS RF switches to be dramatically reduced while maintaining excellent RF performance, so that they can be used more broadly for mobile wireless communication systems, including broadband satellite communications and broadband cellular communications, and thereby be available for use by a wide consumer base.
The present invention, which uses any single or combination of MEMS components, including: electronically-tunable variable capacitors, closed-loop controlled variable capacitors, tunable inductors, tunable LC filters, tunable LC networks, fabricated on an LTCC substrate, allows the cost of MEMS devices to be dramatically reduced while simultaneously achieving high performance and functionality so that these devices and systems can be more broadly used for mobile wireless communication systems, broadband satellite communications or broadband cellular communications, and thereby be available for use by a wide consumer base.
The present invention, which uses MEMS RF switches and/or phase shifters fabricated on an LTCC substrate, allows the cost of phased-array antennas to be dramatically reduced so that phased-array antennas can be more broadly used for mobile wireless communication systems, including broadband satellite communications and broadband cellular communications, and thereby be available for use by a wide consumer base.
Furthermore, the present invention also enables the fabrication and packaging of other micromechanical and microelectronic components, either discrete or integrated, onto substrate materials which have high-performance characteristics at elevated operational frequencies and low-cost.